IRA copy heuristics

[Vladimir Makarov]

David Edelsohn wrote:
> On Thu, Sep 4, 2008 at 7:39 PM, Vladimir Makarov <vmakarov@redhat.com> wrote:
>   
>> Meanwhile I am going to submit your second patch with an added
>> comment.  The patch permits gcc to generate the same quality code as
>> before your first patch.
>>     
>
> Why?
>
> As Richard said before:
>
> "... it changes
> the heuristics _without any explanation of why this is necessary_.
> IMO, that's unacceptable for our shiny, new (and generally very nice)
> register allocator.  And I think it's unacceptable even if it happens
> to fix a performance regression."
>
> The patch does not seem to have any justification other than it fixes the
> symptom.
>
>
>   
  It fixes the performance degradation.  I wrote a long comment for the 
macro about necessity of the problem investigation and marked it by 
???.  The problem is known.  It is actually three monts old (that is 
when Richard submitted the patch first time).  I've tried  to find a 
reason for the degradation which is not easy when you are trying to do 
this on SPEC programs but failed.  Richard found one problem on a small 
test he thought that it could be a reason.  Most part of the problem is 
solved now, so I doubt that was a real reason.

Now I am really busy with fixing IRA bugs.  So if it is ok to gcc 
community to wait for month or two (but may be it will take less who 
knows) to find a real reason or explanation of the performance 
degradation, it is ok for me too.